Thermostatic expansion valves are used to control or meter the flow of refrigerant to an evaporator in an air conditioning system, to provide a refrigerant flow rate into the evaporator that approximately matches the refrigerant flow rate exiting the evaporator. The refrigerant flowing through the thermostatic expansion valve experiences an expansion and a drop in pressure, which results in a refrigerant vapor being supplied to the evaporator. The vapor is then superheated in the evaporator before it enters the suction inlet to the compressor of the air conditioning system.
The typical thermostatic expansion valve operates via a working fluid having a “charge” pressure that changes in response to sensing the temperature of the refrigerant suction line to the compressor. The working fluid pressure acts against a diaphragm in the thermostatic expansion valve to effect opening and closing of a valve. By controlling the refrigerant flow to the evaporator, the thermostatic expansion valve maintains a predetermined amount of superheat in the evaporator to ensure that only vapor is leaving the evaporator. If there is insufficient refrigerant or superheat in the evaporator, un-evaporated liquid refrigerant leaving the evaporator could enter the suction inlet to the compressor. Liquid refrigerant entering the suction inlet to the compressor could cause overheating or damage to the compressor.